[Issue 18024] checkedint.Warn should be @safe

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18024

--- Comment #5 from Seb <greensunny12 at gmail.com> ---
> Don't forget it also prints to stderr, which is why it's @system impure gc and throws

Yeah I did forget about this when I opened the issue (didn't had time to dive
into this then).
Anyhow it `Warn` can easily be made @safe:

PR https://github.com/dlang/phobos/pull/5928

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